The soul is independent of memories, but memories form a part of the soul. Kayden discovered this after many years of studying his soul. She hadn't had many differences since he regained his memories. This actually had a self-evident reason to him: the memories had never disappeared, they had just suffered a powerful attack and were cloudy; the memory-soul conformation was something ridiculously complex.
While the two were different things, they complemented each other. Maybe that was why no soul was destroyed; most of the time, they just reincarnated without any memory and perpetuated the cycle. Kayden could freely touch his memories with the control he gained over his soul. It was an absurd power, as he could repeat experiences until he understood them or saw details he had never seen before.
